Flooding risk increases

Recent snowfall raises concern

March 22, 2013

The Souris River is expected to remain within its banks where the height has been improved by diking, but many rural areas along the Souris are now projected to exceed flood stage this spring.

The risk of overland flooding has also increased from what was projected in the March 7 Flood Potential Outlook issued by the National Weather Service. The latest outlook was released Thursday.

Allen Schlag, NWS hydrologist in Bismarck, cited two key reasons for changes to the most recent outlook.

Article Photos

Kim Fundingsland/MDNHow high this time? In this May 25, 2011, photo, a worker checks on the progress of dike building and sandbagging along Fourth Avenue Northeast where it passes underneath the Third Street Viaduct. Note the high level of water in the Souris River several weeks before severe flooding began in the city.

Submitted photoThe first U.S. National Hydrologic Assessment of the season was issued by the National Weather Service Thursday. As this graphic shows, there is concern over the potential flood risk along the Souris River in north-central North Dakota and the Red River that forms the state’s eastern boundary.

"First and foremost, you've gotten more snow. Your water equivalents have gone up," said Schlag. "You had two and one-half to three inches of water, now it's four to four and one-half. That's a pretty significant rise in water content over the last few weeks. Another important change is that late Tuesday afternoon the International Souris River Board declared a flood year along the Souris River."

Each year the ISRB meets to determine whether or not to declare a 1-in-10 flood event for the Souris. During their meeting in Winnipeg last month the board declared conditions throughout the basin did not warrant a 1-in-10 declaration. They reversed that decision earlier this week based on increased estimates of runoff volume now expected to pass through the Souris River at Sherwood.

With a 1-in-10 declaration in effect, the U.S. Army Corps of Engineers assumes operation of the Souris River system, including deciding how much water is to be released from Lake Darling Dam. The amount of water being released from Lake Darling was increased earlier this week from 250 cubic feet per second to 525 cfs. While flows of 525 cfs are not considered far from the ordinary for this time of year, the effect will be an increased weakening, perhaps even a clearing out, of Souris River ice.

The increased releases from Lake Darling roughly match the amount of water being released from two reservoirs in Saskatchewan. As of Thursday, Rafferty Reservoir near Estevan was releasing approximately 353 cfs. Alameda, located on Moose Mountain Creek a short distance above its confluence with the Souris, was releasing 53 cfs.

Lake Darling stood at 1,595.69 feet, declining late Thursday. The level is slightly below the facility's winter operating level of 1,596 feet. The Saskatchewan reservoirs remain within the operating level guidelines specified by the ISRB for flood control.

As for the latest Flood Potential Outlook, numbers reflect the increased chances of flooding in rural areas along the Souris. Currently the greatest areas of concern are from Baker's Bridge to Burlington and at various points downstream of Minot.

The NWS says the Souris has a 95 percent chance of reaching 11.2 feet at Baker's Bridge. Flood stage there is 10 feet. Water begins to flow over the roadway at Baker's Bridge at 14 feet.

Other gauging areas on the Souris rated a 95 percent chance of reaching or exceeding flood stage are Towner, Bantry and Westhope. Although gauging stations bear their names, those communities are not located directly on the Souris. Willow Creek is given a 95 percent chance of reaching 12.2 feet. Flood stage there is 10 feet. Major problems begin to occur at levels over 15 feet.

The current projection for Minot's Broadway Bridge is a 50 percent chance of 1,545.2 feet and a 95 percent chance of 1,544.2 feet. Flood stage at Broadway Bridge is 1,549 feet.

"Probably the best way to look at it is, the water is going to high, clearly," said Schlag. "We still have pretty low probabilities of problematic high water. The rural areas with no dikes is our biggest concern right now. Conditions suggest we are again priming ourselves for what could be another good, solid year of overland flooding."

According to Schlag, township or county roads that went under water in 2009 or 2011 have a good chance of becoming inundated again this year.

"We don't have as much snow as 2011, but we're close enough to see overland problems," concluded Schlag. "There is a fair amount of concern here."

Schlag said the snowpack along the Souris more closely resembles what normally occurs in January or February, not March or early April. Recent cold temperatures have not prepared, or "ripened", the snowpack for the upcoming melt.

"It's rare to see that kind of snow this time of year," cautioned Schlag.

While recent temperatures have been averaging about 15 degrees below normal, Schlag says a "sudden reversal of fortunes" could occur, meaning a period of temperatures ranging up to 15 degrees above normal. Should that happen, it would push the thermometer close to 60 degree daytime temperatures and lead to a very rapid melt. A quick melt generally means an upward revision in river levels.

The Des Lacs River, often a major contributor to flows in the Souris River above Minot, is still not expected to create difficulties once the melt gets under way. That is, of course, if no major precipitation events occur over the Des Lacs River Basin in the interim.

"Our aerial gamma surveys on a number of flight lines in the Des Lacs watershed are lower for water content than what we are seeing over the Souris," noted Schlag.

In the latest outlook the Des Lacs is given a 95 percent chance of reaching 11.2 feet at Foxholm. Flood stage at that location is 16 feet.